Overhead Conductor Motion During Short Circuits

Edward S. Thomas, PE & Richard A. BarberUtility Electrical Consultants, PC

“Serving Utilities Since 1995”

Raleigh, North Carolina

2015 IEEE Rural Electric Power Conference

Asheville, North Carolina

Factors Affecting Conductor Motion

Conductor Unit Weight (lbs. per ft.)

Fault Current in Conductor (amperes)

Duration of Fault Current (seconds)

Conductor Spacing

Conductor Tension & Sag

Mechanical Damping

Larger substations that increase available fault currents.

Closer conductor spacing in order to minimize aesthetic impact of overhead lines.

Increased customer sensitivity to momentary interruptions and voltage dips.

Increased conductor sag due to the use of larger conductors while maintaining

distribution tension limits.

Why Interest now in Conductor Motion?

D = the sag of a level span.

S = the span length.

H = the conductor tension.

w = the unit weight of the conductor.

Figure 1: Catenary Parameters(Equation 1b)

Catenary Parameters and Basic Equation

F = the force in pounds per foot of conductor.

d = the spacing between the conductors in feet.

I = the symmetrical short-circuit current.

(Equation 2)

Magnetic Forces Between Conductors

(Equation 3)

d = the conductor diameter, in.

VW = the wind speed, mph.

FH = the horizontal wind force, lbs/ft.

Wind Forces on Conductors

(Equation 4a)

WC = the conductor weight per unit length, lbs/ft.

(Equation 4b)

XH = the horizontal deflection at midpoint of span, ft.

D = the midpoint sag of conductor at specified wind

and conductor temperature, ft.

Figure 2: Conductor Swing

Conductor Displacement Due to Horizontal Forces

Calculation of Motion

(Equation 4a) (Equation 4b)(Equation 2)

Apply Equations 4a & 4b to determine displacement using initial force based

on conductor ‘at rest’ position with Equation 2.

Calculate displacement for 0.01 seconds.

Apply Equation 2 for new horizontal separation and reiterate with

Equations 4a & 4b.

Continue iterations until limit is reached. Limit is when gravity vector

equals vertical vector component of horizontal force acting on displaced

conductor or horizontal position is reached. Also limit iterations to fault

current duration.

NESC Requirements for Horizontal Spacing

NESC requirements are based principally on clearances to minimize

contact during wind events.

NESC requirements are basically the same as in NBS Handbook 81 (1961).

Importance of Conductor Tension

Typical 250' Span Conductor Final Sag - (IN.)

Conductor Design 60°FInitial # 60°F 90°F 167°F 75% - 60°F

Initial # 60°F 90°F 167°F

1/0 ACSR 1243 910 20.5 29.5 45.6 682 27.1 37.3 52.1

4/0 ACSR 2000 1394 26 36 52 1046 32.8 42.8 58.1

336 ACSR 2000 1172 38 49.1 72.2 879 46.1 56.3 77.8

556 ACSR 2000 1149 54.7 64 83.8 862 69.4 77.2 94

556 ACSR 3000 1812 39.8 50.6 73.4 1360 43.7 54 76.1

Why Worry about 167°F (75°C)?

Conductor Ampacity

Wind Angle Book* 90°** 45°** 0°**

1/0 ACSR 243 198 182 121

4/0 ACSR 366 294 271 180

336 ACSR 519 419 386 262

556 ACSR 711 571 526 369

All values at 75°C (167°F) conductor temperature with 2 FPS

wind.

* 25°C Ambient

** 40°C AmbientValues are combined effect of using 40°C (104°F) ambient and various wind

angles.

Typical RUS Structures

Figure 4A: RUS C1 Figure 4B: RUS DC-C1

Phase-to-Phase faults more critical.

Conductor Size Required for 10,000 AMP Fault Soft Drawn CU - Start Temp 40ºC

Example of Structure for Phase-to-Ground Fault

Figure 5: Conductor Conflict for a C9 STRUCTURE

Conductor Size Required for 14,500 AMP Fault Soft Drawn CU - Start Temp 40ºC

Conductor Temperature Effect

Conductor 3 kA - 58~ 10 kA - 10~

1/0 ACSR @ 90°F 27" 37" (Horiz)

1/0 ACSR @ 75% & 167°F 45" 47" (Horiz)

Time for Maximum Reverse Swing3000 A for 58~

Conductor Return Time (sec.) Displacement (inches)

1/0 ACSR 1.45 - 1.82 27 - 45

4/0 ACSR 1.58 - 1.90 22 - 33

336 ACSR 1.79 - 2.14 25 - 41

556 ACSR 1.99 - 2.32 21 - 28

Conclusions

Consider conductor temperature under load currents when

determining maximum sags.

Consider maximum operating sags and available short circuit

currents when evaluating allowable span lengths, design tensions

and conductor spacing.

Include measurements of actual conductor sag/tension during

inspections of conductor installations.

When investigating the occurrence of apparent miscoordination,

consider the possibility of conductor clash on the source side of

suspected fault locations.